German Disclosure Publication 27 21 719 discloses an automotive planetary transmission which includes three interconnected planetary gear sets. The planet carrier of the first gear set is coupled with the ring gear of the second gear set, and the carrier of the second set is coupled with the ring gear of the third set. The sun gear of the second set is coupled with the sun gear of the third set. Six forward gear ratios and one reverse gear can be obtained with the transmission by using two clutches in the coupling system and three brakes.
The two clutched couplings rest in front of the three planet sets whereby the transmission is of relatively long construction. Three hollow shafts resting coaxially in each other are required to connect the clutched couplings with the corresponding members of the planet sets whereby the supply of the pressure medium to the shifting couplings becomes complicated. Also, at least the first planet set must have a relatively large diameter since the sun gear of the first planet set is arranged on the outer hollow shaft and must, therefore, have a correspondingly large diameter; this could possibly result in unfavorable transmission ratios and gear gradations and requires also a radially large space.
It is the object of the invention to design a planetary transmission of the kind disclosed in the German publication in such a fashion that the transmission requires little space with a relatively simple construction and such that up to seven advance gears and one reverse gear can be obtained with favorable transmission ratios and gear gradations with two clutched couplings and three brakes.
Each planet set thus includes one uncoupled rotatable member. While these members can be theoretically the ring gears or the planet gear carriers, it is preferred that the sun gears be the uncoupled members because this arrangement leads to a particularly simple and compact construction of the transmission.